Plant growth retardants are widely used in ornamental plant production to control excessive vegetative growth, enhance compactness, and improve overall seedling quality. This study aimed to evaluate the regulatory effects of different paclobutrazol (PBZ) doses on seedling morphology, biomass accumulation, and chlorophyll index in Dahlia × hybrida and Matthiola incana. Although the effects of PBZ on growth and physiology have been reported for various ornamental species, information on morphological responses and biomass distribution during the seedling stage in dahlia and stock remains limited. Seeds of Dahlia × hybrida ‘Figaro’ and Matthiola incana StoX® were sown in 384-cell trays filled with a peat: perlite (2:1, v/v) substrate and grown in a ventilated greenhouse. Following the emergence of the first true leaves, PBZ was applied as a foliar spray at concentrations of 2, 4, 8, 12, and 16 ppm (control: distilled water). Measurements were conducted 40 days after treatment. Seedling height, stem diameter, leaf number, shoot and root fresh and dry weight, root length, and SPAD values were evaluated using a completely randomized design. PBZ treatments were associated with significant variations in seedling growth parameters in both species, with responses differing between species. In dahlia seedlings, 4 ppm PBZ was generally associated with more balanced growth responses, including higher shoot biomass and leaf number, whereas higher doses tended to limit biomass accumulation. In stock seedlings, increasing PBZ doses were associated with reduced seedling height, while higher doses showed higher SPAD values and root length; however, root fresh and dry weights tended to decrease at elevated PBZ concentrations. Overall, the results suggest that PBZ dose optimization is critical for achieving compact growth while maintaining seedling quality, and that species-specific responses should be considered when determining appropriate treatment rates.
Onur Sefa Alkaç (2026) studied this question.